Burner control

ABSTRACT

A control system for an oil burner including a motor for pumping fuel to the burner, an igniter for lighting the fuel, and an electronic switch for controlling energization of the motor and the igniter, all in a line voltage circuit, together with a low voltage circuit for controlling operation of the motor and igniter switch including a relay controlling the gate of the switch, a silicon controlled rectifier and a thermostat in circuit with the relay for energizing the same, a second silicon controlled rectifier for triggering the first mentioned rectifier, and a gate circuit for triggering the second mentioned rectifier including a light sensitive flame detector cell.

United States Patent [1 1 Lenski' 1 Nov. 6, 1.973

[ BURNER CONTROL [75] lnventor: Robert J. Lenski, Rockford, 111.

[73] Assignee: Sundstrand Corporation, Rockford,-

Ill.

[22] Filed: Sept. 25, 1972 [21] Appl. No.: 292,142

[52] U.S. Cl. .L 431/79 51 rm. C1 F23n 5/08 [58] Field of Search 431/79, 24, 78, 75

[56] References Cited UNITED STATES PATENTS 7 3,393,966 7/1968 Giuffrida 431/24 3,453,063 1/1969 Lewis 431/69 3,482,922 12/1969 Blackett 431/79 3,537,804 11/1970 Walbridge 431/79 X 3,672,811 6/1972 I Hrom 431/79 X Primary ExaminerEdward G. Favors AttorneyHofgren,' Wegner, Allen, Stellman &

McCord [57] ABSTRACT A control system for an oil burner including a motor for pumping fuel to the burner, an igniter for lighting the fuel, and an electronic switch for controlling energization of the motor and the igniter, all in a line voltage circuit, together with a low voltage circuit for controlling operation of the motor and igniter switch including a relay controlling the gate of the switch, a silicon con trolled rectifier and a thermostat in circuit with the relay for energizing the same, a second silicon controlled rectifier for triggering the first mentioned rectifier, and a gate circuit for triggering the second mentioned rectifier including a light sensitive flame detector cell.

16 Claims, 2 Drawing Figures 5555-- {7% swing W- PATENTED NOV 6 I973 SHEET 2 OF 2 kkmhgxkwik BURNER CONTROL BACKGROUND OF THE INVENTION The present invention relates to controls for a heating system such as an oil burner of the type utilized in residential buildings, for example, wherein the burner is responsive to a call for heat by a thermostat, which is intended to energize a fuel supply means and an igniter for lighting the fuel.

In systems of the type described, there is usually a relatively delicate balance between fuel supply and air supply within which clean, efficient combustion can be expected. If there is a fuel supply without prompt ignition, or if combustion is initiated and then discontinued inadvertently or unexpectedly, it is important that the fuel supply be discontinued promptly in order to avoid conditions which could be harmful to the equipment or dangerous to property or personnel. Accordingly, it is important in burner control systems to provide safe failure in event the flame does not start'when desired, or goes out after starting.

In the past, control systems of the type described have often utilized electromechanical controls including moving parts which are subject to wear. As controls have become more complicated, numerous moving parts have led to relatively short life and expensive maintenance problems. Also, the controls have often been incorporated in line voltage circuits which may be dangerous and destructive of circuit components in event of malfunction.

Recently, there have been some controls utilizing electronic components in low voltage circuits in an effort to improve safety and reliability; For example, U.S. Pat. No. 3,624,407 relates to a furnace control with electronic components in a low voltage circuit, but the short time, the safety switch opens the control circuit to prevent energization of the fuel supply and igniter, and the system cannot be placed in operation again without manually resetting the safety switch.

In the low voltage control circuit, a relay for energizing the burner means is connected in circuit with the thermostat, the safety switch, and an electronic triggeringswitch, the gate for the electronic switch is connected in circuit with a second electronic switch and a heater for the safety switch, and the gate for the second electronic switch is in circuit with the flame detector, so that in the absence of a flame when the thermostat is calling for heat, the electronic switches are conductive, to energize the relay, and when a flame is detected, the second switch is non'conductive and the heater is deenergized.

Preferably, the electronic switches in the low voltage control circuit are silicon controlled rectifiers so that the relay remains energized after triggering,'so long as the thermostat calls for heat and the power supply is maintained. There is no need for separate energizing means and holding means for the relay.

When the system is placed in operation, the burner has constant ignition, because the igniter is wired in patented control includes a reed switch controlling the burner which requires separate energizing and holding coils. U.S. Pat. No. 3,672,811 also relates :to a burner control, but essentially all of the control components are included in a line voltage circuit, and the fuel supply is controlled by a light sensitive relay which requires both an activating lamp and a holding lamp.

It is desirable to provide an improved burnercontrol system with the control components in a low voltage circuit of simple and reliable construction with fail-safe capacity assuring that the fuel supply is discontinued in the event that there is no ignition or in the event that combustion is unexpectedly discontinued.

SUMMARY OF THE PRESENT INVENTION The present invention relates to an improved burner control system in which a thermostat, flame sensor and safety switch are connected in a low voltage solid state circuit, completely isolated from the commercial 110 volt AC power supply, in an arrangement which provides a safe and electric shock-proof circuit for all external wiring to the control. The system will withstand shorting of any of the thermostat and flame sensor terminals without serious harm to the control.

Only a burner motor, for supplying air and fuel, and an igniter, together with a control switch for the motor and igniter, are included in a line voltage circuit.

When the thermostatic switch closes, calling for heat, the controls in the low voltage control circuit function sequentially to verify operation of a flame detector cell and a safety switch before energizing the fuel supply and igniter means. If a flame is not established in a parallel with the burner motor which supplies fuel and air for combustion. In the event of a power failure, the burner control will remain ready to restart the burner when power is resumed.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, a'line voltage circuit includes a line 20 and line 21 connected across the primary winding of a transformer 22. A burner motor 24 for supplying oil and air to a combustion chamber, is connected to the line 20 by wire 25 and connected to the line 21 through a fuse 27 (if desired) and a triac switch-T1. An igniter 28 is connected in parallel with the motor 24 so that the motor and the igniter are simultaneously energized.

The triac switch Tl includes a main terminal MT2 connected to the fuse 27,a main terminal MTl connected to the line 21 and a gate G connected to a control circuit for triggering the switch when there is a-call for heat by the thermostat. A resistor R2 is connected between the triac terminals G and MTl to eliminate false triggering of the triac due to excessive leakage currents at high temperature or due to electrically generated stray noise signals. A resistor R1 and a capacitor C1 are connected across the triac terminals MT2 and MTl to limit the rate of rise of voltage across the triac T1 to a safe value when the triac switches from on to off.

The triac T1 is normally in its off or nonconducting state when the thermostat is not calling for heat. The gate G of the triac T1 is biased to render the triac conducting by means of a collector current from a driver transistor T3. The collector of the transistor T3 is connected to the triac gate G by a conductor 30 including a resistor R3. The resistor combination of R3 and R2 provides proper gate bias voltage and current for the triac Tl. The base of the transistor T3 is connected-to the emitter of a phototransistor T2 by a conductor 31, and the emitter of the transistor T3 is connected by a conductor 32 to a conductor 33.

The base of the transistor T2 is connected by a conductor 35 and capacitor C6 to the conductor 33, and the latter is connected by a conductor 36 and diode D1 to a tap on the primary winding of the transformer 22. The collector of the transistor T2 is connected to a resistor R4. The phototransistor T2 is arranged to respond to light from a light emitting diode D3 in a low voltage control circuit responsive to the thermostat. Light from the diode D3 striking the transistor T2 initiates a base current flow in the transistor T2, as a result of which the phototransistor conducts, amplifying the small light generated base current into considerably more emitter current. The emitter current from the transistor T2 flows into the base of the driver transistor T3, causing it to saturate and conduct collector current to trigger the triac Tl.

Diode D1 and a capacitor C2 in the conductor 33 function as a half-wave power supply for the gate trigger circuitry of transistors T2 and T3. Resistor R4 acts as a current limit for transistors T3 and T2.

A resistor R5 is connected between the conductors 31 and 33, and capacitor C6 is provided in the conductor 35. The resistor R5. and thecapacitor C6 insure stabilization of the high gain transistor pair T3 and T2.

A low voltage control circuit is connected across the secondary of the transformer 22. The low voltage circuit includes a thermostat incorporating a normally open thermostatic switch 40 which is adapted to close responsive to decreasing ambient temperature to call for heat. The low voltage circuit also includes a light sensitive cadmium-type photoresistive flame detecting cell 42 located adjacent to a burner 43 so that the cell is responsive to the presence or absence of a flame at the burner. The cell 42 has a relatively high resistance, on the order of 50K ohms, in darkness in the absence of a flame at the burner 43, and a relatively low resistance, from 300 ohms to 2,000 ohms, depending upon the distance of the cell from the flame, when the burner is lighted.

Preferably, the burner control circuitry is'incorporated in an appropriate housing represented at 45, with only the burner motor 24, the igniter 28, the thermostat 40 and the'cad cell 42 outside the housing 45. In order to properly connect the control circuitry to the external components of the system, the housing 45 includes a terminal LG for connection to the power supply line 20, a terminal L for connection to the power supply line 21, and a terminal M for connection with the motor 24 and igniter 28. Additionally, the housing 45 includes terminals T for connection of the thermostat, and terminals F for connection of the flame detector cell 32.

The light emitting diode D3 and the phototran'sistor T2 may be appropriately characterized as a light relay generally designated 46, in which the diode D3 is a controlling relay element and the transistor T2 is a controlled relay element.

in the low voltage control circuit, the light emitting diode D3 is connected to be energized responsive to closure of the thermostatic switch 40, provided that certain other conditions exist, as will appear in the description of the circuitry.

The low voltage circuit includes a safety switch with normally closed switch contacts 47 and a heater coil 48 arranged to cause opening of the contacts 47 after a predetermined period on the order of 15 to 45 seconds. After the switch contacts are opened, they are latched in open condition and must be manually reset by means of a button 49 accessible from the outside of the housing 45. If desired, the physical construction of the safety switch may be on the order of that shown and described in the aforementioned U. S. Pat. No. 3,624,407.

In order to provide for energization of the light emitting diode D3 when there is a call for heat, the diode is connected across the secondary winding of the transformer 22 in a circuit including a conductor 50 leading from the transformer secondary (+12) to the safety switch 47, a conductor 51 leading from the safety switch 47 to the thermostat, a conductor 52 leading from the thermostat, a conductor 54 leading from the conductor 52 to the diode D3, a conductor 55 leading from the diode D3 to an electronic switch T4, preferably a silicon controlled rectifier, and a conductor 56 returning from the SCR T4 to the transformer secondary (0). A resistor R7 is connected across the conductors 54 and 56 and draws current through the thermostat when it is closed, before the SCR T4 conducts. A diode D2 and a resistor R8 are provided in the conductor 54. A conductor 58 is connected to the conductor 54 and includes a power supply capacitor C3. A resistor R9 is connected across the capacitor between the conducter 54 and the conductor 58. A current limiting resistor R10 is provided in conductor 54 in circuit with diode D3.

On the positive half cycle swings of secondary transformer voltage (+12), diode D2 conducts, charging the power supply capacitor C3 to about +17 volts through resistor R8. The capacitor will serve to provide current to the diode D3, when T4 conducts, during negative half cycle swings of the transformer secondary winding. Resistor R9 serves to completely discharge the capacitor C3 after the thermostat opens and T4 switches off to its high impedance state.

In order to provide for triggering the SCR T4, a conductor 60 leads from the safety switch contacts 47 to the heater 48, and a conductor 61 leads from the heater to a second electronic switch T5, also preferably a silicon controlled rectifier. The gate of the SCR T5 is connected by a conductor 63 to the flame detector cell 42, and a conductor 64 connects the cell 42 to the transformer secondary winding (6) through a resistor R16. The cathode of the SCR T5 is connected by a conductor 66 to the gate of the SCR T4, so that the SCR T5 may function to trigger the SCR T4. A capacitor C4 connected to conductor 61 dampens stray noise signals due to contact bounce of the safety switch contacts 47 and the thermostat contacts 40. Similarly, a capacitor C5 connected to the gate of the SCR T5 eliminates stray noise signals at the gate of the SCR T5.

A conductor 68 is connected between the conductor 54 and the conductor 63 and includes a resistor R11, which together with a resistor R14 in the conductor 63 and a grounded resistor R15 form a resistor bias network for controlling the gate of the SCR T5. In the absence of a flame in the burner, the cad cell 42 has a very high resistance on the order of 50K ohms, and such high resistance state causes the gate-cathode junction of the SCR T5 to be forward biased by the resistor network R11, R14 and R during positive half cycle swings of the transformer winding (l2). The SCR T5 receives enough gate current and gate voltage to remain triggered as long as the flame detector cell 42 remains in its darkened state indicating the absence of a flame at the burner 43. v

The triggering of the. SCR T5, as described above, causes it to conduct current through the resistive heating element 48 in the safety switch. The heating element in turn is arranged to heat a bimetallic strip in the 48. Thus, if no flame is established at the burner 43in the 15 to 45 second delay following closure of the thermostat contacts, the safety contacts 47 will open to discontinue power to the low voltage control. Once the safety switch contacts havebeen opened, they remain open, and the entire'control is disabled, until the safety switch is manually reset.

During the 15 to 45 second period required to open the safety switch contacts, SCR T5 conducts current through a resistor R13 and a redundant resistor R6 connected to the conductor 66, so that enough. voltage appears across them to trigger the SCR T4 through its gate and aresistor R12 which controls the current to the gate. When SCR T4 conducts, a circuit is completed through the light emitting diode D3, so that the latter transmits light to energize the burner controls. After triggering, the SCR T4 will remain in its conducting state until the thermostat contacts 40 open, or the safety switch contacts 47 open, or the power is lost at the line voltage supply. a

. If a flame is'eatablished at the burner 43 during the 15 to 45 second trial period before the safety switch contacts 47 open, as will normally occur if the control systemfunctions properly, the resistance in the flame detector cell 42 will be reduced to a low value between 300 and 2,000 ohms,-depending upon the proximity of the cell to the flame. The low resistance in the flame detector cell reverse biases the gate-cathode junction of the SCR T5, thereby turning off the SCR T5. When the switch T5 does not conduct, no power is consumed by the heater element 48 in the safety switch, so that it cools and the contacts 47 remain in the normally closed condition. As a result, the burner will continue to operate as long as the thermostat calls for heat and the flame detector cell reads a flame condition.

The switching devices T1 and T3-T5 may be of the type identified by various manufacturers as follows:

Tl-lT2l0, Q185 T4-C103Y, 2N5060, TlC44 T5-C103Y, TlC44 The diodes D1 and D2 may be of the type identified by manufacturers as follows:

The capacitors C1-C6 have values generally as follows:

Cl-0.l mfd 400V C2-22O mfd 25V C3-l00 mfd 25V C5-0.00l mfd] Ceramic C6-390 pfd Preferably, the light emitting diode D3 and the phototransistor T2 are combined in a single commercially available integrated circuit such as Monsanto Companys photocoupler MCT 26.

The resistances Rl-R16 have values approximately as follows:

R6-4.3 ohm VaW R95.6K ohm TO 10K ohm 56W RID-560 ohm AW Rll-lSK ohm AW R12-330 ohm AW R13-4.3 ohm AW R14-2.2K ohm SW To summarize the operation, when there is no call for heat, the thermostatic switch contacts 40 are normally open, and the components of the system are at rest. When the thermostatic switch is closed, a circuit is completed through the conductor 68 to the gate circuit for the SCR T5. If the flame detector cell is functioning properly, when there is no flame the high resistance at the cell causes theSCR T5 to conduct, for purposes of triggering the SCR T4. A circuit is completed through the safety switch contacts 47 and the thermostat to energize the light emitting diode D3 when the SCR T4 is conducting. Energization of the diode D3 results in transmission of light to the phototransistor T2. The emitter current from the phototransistor T2 causes the transistor T3 to conduct. The collector current from the transistor T3 biases the gate G of the triac T1 so that the latter is rendered conducting, to'energize the burner motor 24 and the igniter 28. The motor drives a pump for supplying fuel oil to the burner 43 and the igniter causes ignition of such fuel. The existence of a flame reduces the resistance in the flame detector cell 42, rendering the SCR T5 nonconducting. The heater coil 48 is deenergized, and the safety switch contacts 47 remain closed.

If for some reason, the normally expected cycle of operation described above is not completed, and no flame is established at the burner 43, the high resistance in the unlighted flame detector cell 42 will maintain the SCR T5 conducting until the heater element 48 opens the safety switch contacts 47, thereby shutting the system down until it receives manual attention.

Among the advantages of the system illustrated, the flame sensor and the thermostat are connected to low voltage circuits completely isolated from the volt commercial power supply, by virtue of the'light sensitive relay providing a photoelectric coupling between the low voltage circuit and the line voltage circuit. This results in a shock-proof arrangement for external wiring to the control. The photo-electric relay coupling the low voltage circuit and the line voltage circuit is composed of highly reliable solid state devices with extremely long life expectancy. The photoelectric triac gate triggering circuit provides an improvement over previous controls, in that the circuit is isolated from the motor and igniter by virtue of a separate tap on the transformer winding, so that the circuit is not affected by variations in the characteristics of the windings in the motor and igniter. The isolated photoelectric triggering circuit insures good starting of the motor at low temperatures, when more triac gate current may be required, by providing full 360 A.C. sine wave conduction. The circuit also eliminates false starts at high temperatures due to leakage currents.

The arrangement illustrated provides for safe failure. If a malfunction condition exists such that the thermostat terminals T are shorted, and the flame sensor terminals F are open, the burner control may turn on, but it will not operate longer than the time required for the heater 48 to open the safety switch contacts 47, because there will be no reduction of resistance to render the SCR T non-conducting. If the flame sensor terminals F are shorted, the burner control will never start, because the SCR T5 will be nonconducting. This sequential safety check insures that the flame detector is functioning properly before the system is put into operation when the thermostat calls for heat. In event the flame sensor terminals are shorted to the thermostat terminals T, resistor R16 limits the current in the transformer secondary to a safe value.

The igniter 28 is constantly energized during operation of motor 24. If there is a power failure, the control will restart the system when power returns.

FIG. 2

The embodiment of FIG. 2 is similar to the embodiment of FIG. 1, except that the photoelectric relay 46 utilized in the control of FIG. I is omitted, along with the circuit for amplifying the photoelectric signal, and in place thereof an electromagnetic relay 70 is utilized including a coil 72 and switch contacts 74. The remaining components of the system in FIG. 2 correspond substantially to similar components in FIG. 1 and are identified. by similar-reference numbers primed. The coil 72 functions as a controlling relay element'in place of light emitting diode D3, and the switch 74 functions as a controlled relay element connected directly to the gate terminal G of triac T1, in lieu of phototransistor T2 and its amplifying circuit. A conductor 75 leads from fuse 27 through the relay switch 74 to the gate terminal G for triac Tl.

The circuit has the advantage of reducing the control elements associated with the triac switch controlling the burner motor and igniter. The switch 74 may be a coil controlled magnetically sensitive mercury switch of a type'manufactured by Fifth Dimension, Inc., of Princeton, N..I., and called Logcell. In the Logcell switch, an elongate core for the coil has a mercury coated terminal at one end in a sealed cap containing a flexible spring disc contact engageable with the terminal to close a circuit responsive to energization of the coil. The circuit of FIG. 2 has the reliability, shockproof and fail-safe characteristics as described in connection with FIG. 1.

I claim:

1. A burner control system comprising:

a. means providing a line voltage circuit,

b. burner means in the line voltage circuit adapted to be selectively energized,

c. a first electronic switch in circuit with the burner means,

d. a circuit for triggering said switch including a controlled relay element,

e. means providing a source of low voltage including a normally closed safety switch,

f. a controlling relay element in circuit with the low voltage source for controlling said controlled relay element,

g. a thermostat in circuit with said controlling relay element for conditioning the circuit responsive to to a call for heat,

h. a second electronic switch in circuit with said controlling relay element and thermostat for energizing said controlling relay element,

i. a circuit for triggering said second electronic switch including a third electronic switch in circuit with the low voltage source,

j. a heater for said safety switch in circuit with said third electronic switch,

k. a gate circuit for triggering said third electronic switch in circuit with said low voltage source and said thermostat, and

l. a light-sensitive flame detector in the gate circuit for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostat is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive when 'the burner is lighted.

2. A burner control system as defined in claim 1, wherein the burner means comprises a motor for supplying fuel and an igniter for lighting the fuel.

3. A burner control system as defined in claim I, wherein the flame detector comprises a cadmium type photosensitive cell.

4-. A burner control system as defined in claim 1, wherein the first electronic switch comprises a Triac including one main terminal connected to the burner means, another main terminal connected to the line voltage circuit, and a gate for rendering the switch conductive across the main terminals when the gate is energized.

5. A burner control as defined in claim 1, wherein the second electronic switch comprises a silicon controlled rectifier having an anode, a cathode, and a gate for triggering the switch, and remains conductive after the triggering circuit therefor is deenergized, while the thermostat remains closed calling for heat.

6. A burner control system as defined in claim 1, wherein the third electronic switch comprises a silicon controlled rectifier having an anode, a cathode and a gate for triggering the switch.

7. A burner control system as defined in claim 1, wherein the controlled relay element comprises a switch and the controlling relay element comprises an electromagnetic coil.

8. A burner control system as defined in claim I, wherein the controlled relay element comprises a light sensitive element and the controlling relay element comprises a light transmitting element.

9. A burner control system as defined in claim 8, wherein said light sensitive element comprises a phototransistor.

10. A burner control system as defined in claim 8,-

wherein the light transmitting element comprises a light emitting diode.

11. A burner control system comprising:

a. means providing a line voltage circuit including selectively energizable burner means,

b. a first electronic switch in circuit with the burner means including a gate for triggering said switch, c. a light-sensitive element in circuit with the gate for rendering the gate conductive, I

d. means providing a source of low voltage,

e. a light emitting element for transmitting light to said light-sensitive element,

f. a thermostat in circuit with said low voltage source and said light emitting element for conditioning the circuit responsive to a call for heat,

I g. a second electronic switch in circuit with said light emitting element and thermostat for energizing said light emitting element including a gate for triggering said second electronic switch,

h. a third normallynonconductive electronic switch in circuit with the low voltage source for rendering the last recited gate conductive, 4

i. a gate for triggering said third electronic switch in circuit with said low voltage source and said thermostat, and

j.- a light-sensitive flame detecting cell in circuit with the gate for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostat is closed, and provide a relatively .low resistance in the presence of a burner flame to render the third switch nonconductive when the burner is lighted.

12. A burner control system comprising:

a. means providing a line voltage circuit,

b. means in the line voltage circuit for supplying fuel and igniting a burner,

c. a first normally nonconductive electronic switch in circuit with the fuel supply and ignition means,

d. a gate circuit for triggeringsaid switch including a light-sensitive element for rendering the gate circuit conductive, I I V e. means providing a source of low voltage including a normally'closed safety switch,

f. a light emitting element in circuit with the low voltage source for transmitting light to said lightsensitive element,

g. a thermostatic switch in circuit with said light emitting element adapted to close for conditioning the circuit responsive to a call for heat,

h. a second normally nonconductive electronic switch in circuit with said light emitting element and thermostatic switch for energizing said light emitting element,

i. a gate circuit for triggering said second electronic switch including a third normally nonconductive electronic switch in circuit with the low voltage source,

j. a heater for said safety switch incircuit with said third electronic switch,

k. a gate circuit for triggering said third electronic switch in circuit with said low voltage source and said thermostatic switch, and

l. a light sensitive flame detector in the gate circuit for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostatic switch is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive when the burner is lighted.

13. A burner control system comprising:

a. means providing a line voltage circuit,

b. means in the line voltage circuit energizable for supplying fuel and igniting a burner,

c. a triac switch in circuit with the fuel supply and ignition means,

d. a circuit for triggering said triac switch including a light-sensitive phototransistor for rendering the circuit conductive,

e. means providing a source of low voltage including normally closed safety switch contacts,

f. a light emitting diode in circuit with the low voltage source for transmitting light to said phototransistor, I

g. a thermostat in circuit with said light emitting element for conditioning the circuit responsive to a call for heat,

h. a first silicon controlled rectifier in circuit with light emitting diode and thermostat for energizing said light emitting diode,

i. a circuit for triggering said first silicon ontrolled rectifier including a second silicon controlled rectifier in circuit with the low voltage source,

j. a heater for said safety switch contacts in circuit with said second silicon controlled rectifier,

k. a circuit for triggering said second silicon controlled rectifier in circuit with said low voltage source and said thermostat, and

. a light-sensitive cadmium type flame detecting cell in the gate circuit for the second silicon controlled rectifier normally providing a relatively high resistance to trigger said second silicon controlled rectifier in the absence of a burner flame when the thermostat is closed, and providea relatively low resistance in the presence of a burner flame to render the second silicon controlled rectifier nonconductive' when the burner is lighted.

14.. A burner control system comprising:

a. means providing a line voltage circuit,

b. means in the line voltage circuit for supplying fuel and igniting a burner,

c. a first normally nonconductive electronic switch in circuit with the fuel supply and ignition means,

(1. a gate circuit for triggering said switch including switch contacts for rendering the gate circuit conductive, 4

e. means providing a source of low voltage including a normally closed safety switch, I

f. an electromagnetic coil in circuit with the low voltage source for controlling said switch contacts,

g. a thermostatic switch in circuit with said coil element adapted to close for conditioning the circuit responsive to a call for heat,

h. a second normally nonconductive electronic switch in circuit with said coil and thermostatic switch for energizing said coil,

i. a gate circuit for triggering said second electronic switch including a third normally nonconductive electronic switch in circuit with the low voltage source,

j. a heater for said safety switch in circuit with said third electronic switch,

k. a gate circuit for triggering said third electronic switch in circuit with said low voltage source and said thermostatic switch, and

l. a light-sensitive flame detector in the gate circuit for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostatic switch is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive switch including a second normally nonconductive electronic switch in circuit with the low voltage source,

f. a heater for said safety switch in circuit with said second electronic switch,

g. a gate circuit for triggering said second electronic switch in circuit with said low voltage source and said thermostatic switch, and

h. a light-sensitive flame detector in the gate circuit for the second electronic switch normally providing a relatively high resistance to bias said second switch to conduct in the absence of a burner flame when the thermostatic switch is closed, and provide a relatively low resistance in the presence of a burner flame to reversely bias the second switch to a nonconductive state when the burner is lighted.

16. A burner control system as defined in claim 12,

wherein the gate circuit for the first electronic switch is a low voltage circuit independent of the fuel supply and ignition means so that triggering is not affected by variation in electrical characteristics of the fuel supply and ignition means. 

1. A burner control system comprising: a. means providing a line voltage circuit, b. burner means in the line voltage circuit adapted to be selectively energized, c. a first electronic switch in circuit with the burner means, d. a circuit for triggering said switch including a controlled relay element, e. means providing a source of low voltage including a normally closed safety switch, f. a controlling relay element in circuit with the low voltage source for controlling said controlled relay element, g. a thermostat in circuit with said controlling relay element for conditioning the circuit responsive to to a call for heat, h. a second electronic switch in circuit with said controlling relay element and thermostat for energizing said controlling relay element, i. a circuit for triggering said second electronic switch including a third electronic switch in circuit with the low voltage source, j. a heater for said safety switch in circuit with said third electronic switch, k. a gate circuit for triggering said third electronic switch in circuit with said low voltage source and said thermostat, and l. a light-sensitive flame detector in the gate circuit for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostat is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive when the burner is lighted.
 2. A burner control system as defined in claim 1, wherein the burner means comprises a motor for supplying fuel and an igniter for lighting the fuel.
 3. A burner control system as defined in claim 1, wherein the flame detector comprises a cadmium type photosensitive cell.
 4. A burner control system as defined in claim 1, wherein the first electronic switch comprises a Triac including one main terminal connected to the burner means, another main terminal connected to the line voltage circuit, and a gate for rendering the switch conductive across the main terminals when the gate is energized.
 5. A burner control as defined in claim 1, wherein the second electronic switch comprises a silicon controlled rectifier having an anode, a cathode, and a gate for triggering the switch, and remains conductive after the triggering circuit therefor is deenergized, while the thermostat remains closed calling for heat.
 6. A burner control system as defined in claim 1, wherein the third electronic switch comprises a silicon controlled rectifier having an anode, a cathode and a gate for triggering the switch.
 7. A burner control system as defined in claim 1, wherein the controlled relay element comprises a switch and the controlling relay element comprises an electromagnetic coil.
 8. A burner control system as defined in claim 1, wherein the controlled relay element comprises a light sensitive element and the controlling relay element comprises a light transmiTting element.
 9. A burner control system as defined in claim 8, wherein said light sensitive element comprises a phototransistor.
 10. A burner control system as defined in claim 8, wherein the light transmitting element comprises a light emitting diode.
 11. A burner control system comprising: a. means providing a line voltage circuit including selectively energizable burner means, b. a first electronic switch in circuit with the burner means including a gate for triggering said switch, c. a light-sensitive element in circuit with the gate for rendering the gate conductive, d. means providing a source of low voltage, e. a light emitting element for transmitting light to said light-sensitive element, f. a thermostat in circuit with said low voltage source and said light emitting element for conditioning the circuit responsive to a call for heat, g. a second electronic switch in circuit with said light emitting element and thermostat for energizing said light emitting element including a gate for triggering said second electronic switch, h. a third normally nonconductive electronic switch in circuit with the low voltage source for rendering the last recited gate conductive, i. a gate for triggering said third electronic switch in circuit with said low voltage source and said thermostat, and j. a light-sensitive flame detecting cell in circuit with the gate for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostat is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive when the burner is lighted.
 12. A burner control system comprising: a. means providing a line voltage circuit, b. means in the line voltage circuit for supplying fuel and igniting a burner, c. a first normally nonconductive electronic switch in circuit with the fuel supply and ignition means, d. a gate circuit for triggering said switch including a light-sensitive element for rendering the gate circuit conductive, e. means providing a source of low voltage including a normally closed safety switch, f. a light emitting element in circuit with the low voltage source for transmitting light to said light-sensitive element, g. a thermostatic switch in circuit with said light emitting element adapted to close for conditioning the circuit responsive to a call for heat, h. a second normally nonconductive electronic switch in circuit with said light emitting element and thermostatic switch for energizing said light emitting element, i. a gate circuit for triggering said second electronic switch including a third normally nonconductive electronic switch in circuit with the low voltage source, j. a heater for said safety switch in circuit with said third electronic switch, k. a gate circuit for triggering said third electronic switch in circuit with said low voltage source and said thermostatic switch, and l. a light sensitive flame detector in the gate circuit for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostatic switch is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive when the burner is lighted.
 13. A burner control system comprising: a. means providing a line voltage circuit, b. means in the line voltage circuit energizable for supplying fuel and igniting a burner, c. a triac switch in circuit with the fuel supply and ignition means, d. a circuit for triggering said triac switch including a light-sensitive phototransistor for rendering the circuit conductive, e. means providing a source of low voltage including normally closed safety switch contacts, f. a light emitting diode in circuit with the low voltage source fOr transmitting light to said phototransistor, g. a thermostat in circuit with said light emitting element for conditioning the circuit responsive to a call for heat, h. a first silicon controlled rectifier in circuit with light emitting diode and thermostat for energizing said light emitting diode, i. a circuit for triggering said first silicon ontrolled rectifier including a second silicon controlled rectifier in circuit with the low voltage source, j. a heater for said safety switch contacts in circuit with said second silicon controlled rectifier, k. a circuit for triggering said second silicon controlled rectifier in circuit with said low voltage source and said thermostat, and l. a light-sensitive cadmium type flame detecting cell in the gate circuit for the second silicon controlled rectifier normally providing a relatively high resistance to trigger said second silicon controlled rectifier in the absence of a burner flame when the thermostat is closed, and provide a relatively low resistance in the presence of a burner flame to render the second silicon controlled rectifier nonconductive when the burner is lighted.
 14. A burner control system comprising: a. means providing a line voltage circuit, b. means in the line voltage circuit for supplying fuel and igniting a burner, c. a first normally nonconductive electronic switch in circuit with the fuel supply and ignition means, d. a gate circuit for triggering said switch including switch contacts for rendering the gate circuit conductive, e. means providing a source of low voltage including a normally closed safety switch, f. an electromagnetic coil in circuit with the low voltage source for controlling said switch contacts, g. a thermostatic switch in circuit with said coil element adapted to close for conditioning the circuit responsive to a call for heat, h. a second normally nonconductive electronic switch in circuit with said coil and thermostatic switch for energizing said coil, i. a gate circuit for triggering said second electronic switch including a third normally nonconductive electronic switch in circuit with the low voltage source, j. a heater for said safety switch in circuit with said third electronic switch, k. a gate circuit for triggering said third electronic switch in circuit with said low voltage source and said thermostatic switch, and l. a light-sensitive flame detector in the gate circuit for the third electronic switch normally providing a relatively high resistance to trigger said third switch in the absence of a burner flame when the thermostatic switch is closed, and provide a relatively low resistance in the presence of a burner flame to render the third switch nonconductive when the burner is lighted.
 15. A burner control system comprising: a. means providing a source of low voltage including a normally closed safety switch, b. a controlling relay element in circuit with the low voltage source, c. a thermostatic switch in circuit with said controlling relay element adapted to close for conditioning the circuit responsive to a call for heat, d. a first normally nonconductive electronic switch in circuit with said controlling relay element and thermostatic switch for energizing said controlling relay element, e. a gate circuit for triggering said first electronic switch including a second normally nonconductive electronic switch in circuit with the low voltage source, f. a heater for said safety switch in circuit with said second electronic switch, g. a gate circuit for triggering said second electronic switch in circuit with said low voltage source and said thermostatic switch, and h. a light-sensitive flame detector in the gate circuit for the second electronic switch normally providing a relatively high resistance to bias said second switch to conduct in the absence of a burner flame when the thermostatic switch is closed, and provide a relatively low resistance in the presence of a burner flame to reversely bias the second switch to a nonconductive state when the burner is lighted.
 16. A burner control system as defined in claim 12, wherein the gate circuit for the first electronic switch is a low voltage circuit independent of the fuel supply and ignition means so that triggering is not affected by variation in electrical characteristics of the fuel supply and ignition means. 